IJSRD - International Journal for Scientific Research & Development Vol. 4, Issue 04, 2016 ISSN (online): 2321-0613 Thermal Protection of Induction Motor using DSP Dipak Banbakode 1 Millan Sabat 2 Tanmay Tandel 3 Atul Gupta 4 1,2,3 Department of Electrical Engineering 1 VJTI, Mumbai, India 2 SPEC, Mumbai, India 3 COEP, Pune, India 4 Larsen & Tourbo, Mumbai, India Abstract Induction motor is widely used in the industry. It s downtime in factory can be avoided by providing protection to it. Early detection of abnormalities in the motors will help to avoid expensive failures. Thermal protection is one of the most important features of the motor condition monitoring system. Thermal protection of induction motor using DSP TMS320F28335 is implemented in this paper. The method for thermal protection of induction motor is developed, simulated in MATLAB/Simulink and results are presented. Since it is possible to control the speed of asynchronous motor, its cooling is also varies with the speed of rotor which is considered here. This method calculates thermal capacity accumulated by the motor in terms of percentage. Motor thermal capacity is monitored continuously using motor terminal quantities. If thermal capacity reaches to 100% then trip signal is given to motor. Key words: 3 Phase Induction Motor, Thermal Protection, Thermal Time Constant I. INTRODUCTION Induction motor is one of the most widely used motor in the industry for converting electrical energy to mechanical energy. It is used in industries, in household appliances, and in laboratories. The major reason behind popularity of the induction motors is that they are cheap, rugged in construction, low maintenance cost, high efficiency, high starting torque. Also it is easy to control its speed. Since it is widely used, downtime in a factory can be very expensive and in some instances, may exceed the cost of motor replacement. So its protection is very much important. Early detection of abnormalities in the motors will help to avoid expensive failures. Condition monitoring for proactive monitoring and reliable protection of induction motor has experienced a fast growth in recent years. Thermal protection is one of the most important features of the motor condition monitoring system. The thermal overload of a motor can lead to deterioration of the key components of the motor, including stator winding insulation, bearing, motor conductors, core etc, and therefore is one of major underlying root causes of motor failures. When motor is started from rest, then it will stressed in many ways. Sudden high inrush current causes high electromagnetic forces tending to tear the winding apart, also cause thermal expansion in the winding. Exceeding the number of starts will reduce motor life on the average. Hence motors have a limit on the number of attempted motor starts. Motor winding insulation failure is one of the most common failures for induction motor. About 35-40% of induction motor failures are related to winding insulation failure [1, 2, 3]. The insulation failure is often the result of long-term thermal aging [ 4 ]. It is estimated that motor s life is reduced by 50% for every 10 C increase above the stator winding temperature limit. The typical thermal limits of the stator winding for different insulation classes are listed in Table 1. Rated Hotspot Ambient Hot spot temperatu temperatu temperatu temperatu Insulati re rise re rise re (degree re (degree on class (degree (degree centigrad centigrad centigrad centigrad e) e) e) e) A 40 60 5 105 B 40 80 10 130 F 40 105 10 155 H 40 125 15 180 Table 1: Temperature limits for insulation class [5] Temperature sensors are embedding into the motor winding for temperature monitoring purposes. However, it is not feasible to install temperature sensors in the rotors for technical reasons, reliability and cost [6,7]. Also, this temperature sensor like RTD (resistance temperature detector) has relatively slow reaction time. They are not suitable for the fast thermal transients. Today in the age of competition, various methods for motor thermal protection without using sensors are preferred. DC injection based method for stator resistance estimation is available but this causes torque pulsation and more heating [8]. Since stator resistance varies with stator temperature, the measurement of stator resistance will give temperature of stator winding. By continuously monitoring stator resistance, stator winding temperature will be monitored for motor protection. Slip dependent thermal protection considers the effect of slip on rotor resistance [9]. It is because rotor resistance at start or stall condition is maximum as compared to running condition. In this paper, motor thermal protection is achieved by using only motor terminal quantities, is explained in the following section. II. PROPOSED METHOD In this paper, V/F control method is implemented. This profile is generated in DSP TMS320F28335 using VHZ_PROFILE module available in digital motor control library [10, 11]. This module generates an output command voltage for a specific input command frequency according to the specified volts/hertz profile. Space vector modulation (SVM) technique is used. SVM gives 15.5% more voltage than sine PWM. As SVM is generating more voltage than sine PWM, SVM is capable for driving higher torque load and even fluctuation in input voltage will disturb lesser than sine PWM. The first order differential equation is implemented in this paper for thermal protection of asynchronous motor is [13, 14] I I TC(n 1) TC(n) = TC(n - 1) + [ ] * Δt...(1) Tth Where, TC(n) is the temperature expressed in units of I 2 at sample n, All rights reserved by www.ijsrd.com 824
TC(n-1) is the temperature expressed in units of I 2 at the previous sample, I is the rms value of current, Δt is the updated time interval, Tth is the thermal time constant of motor. DSP require few parameters to be set so as to protect the motor from temperature damage. A. Setting Thermal Time Constant The technical definition of thermal time constant is, "time required for a thermistor to change 63.2% of the total difference between its initial and final body temperature when subjected to a step function change in temperature, under zero power conditions". The motor thermal time is specific to the motor design and it varies between different motor manufacturers. Setting of thermal time constant is done as follows: 1) If IEC Class is known The motor can be protected as defined in the IEC 60947-4-1 standard [12] for the thermal overload relays. If the protection class is known, then value of thermal time constant in seconds can be entered as: IEC Class Thermal time constant [sec] 10 360 20 720 30 1080 Table 2: Setting of thermal time constant Thermal time constant = IEC Class x 36. The standard above defines a 7.2 ratio between LRC and FLC. If the ratio between LRC and FLC is not 7.2, then following (b, c) is referred. 2) If IEC class is not known If the IEC class is not known, then the IEC class can be approximated if the following values are known to us: Full Load Current (FLC) of the motor Locked Rotor Current (LRC) Maximum Locked Rotor Time (LRT) or Direct On Line (DOL) Start Time The FLC of the motor can be obtained directly from the nameplate on the motor. The LRC and LRT must be obtained from the manufacturer or from the motor datasheet. The LRC, also referred to as starting current or motor start-up current, is the current that a motor draws at start-up when full voltage is applied to the stator. This information might also be available as a thermal withstand curve or a thermal damage curve. If this is the case, then the LRC and LRT must be deduced from the curves. The following formula can be applied:...(2) Once the approximated IEC class has been calculated, use the motor thermal time constant that corresponds to the closest IEC class. 3) Enhanced Method If ratio between LRC and FLC is different from 7.2, refer to the graph below, to get more precise calculation. X axis shows the LRC/FLC ratio, and the Y axis shows the multiplicative constant to be applied to the LRT to calculate the value of parameter thermal time constant. Fig. 1: Set up of parameter thermal time constant depending on the LRC/FLC ratio B. Trip Current Setting Parameter trip current is set to 105%. Under the locked-rotor condition, there is normally no ventilation, and the heat loss from the windings is by conduction and radiation. During acceleration, depending on the speed, the heat loss is both by conduction and by the ventilating effect of air movement. During running overloads, the normal ventilation of the machine is the primary mode of cooling. Since with V/F drive, it is possible that even the motor is taking let s say 100% of FLC and its speed may be less than rated speed of any value. So in such cases effect of cooling is different. This is the case when fan is keyed to the shaft. In order to compensate this cooling effect due to speed variation, trip current setting (Itrip) is adjusted accordingly with respect to speed as shown in figure 2. Fig. 2: Trip Current Setting The required parameters which are explained above are fed as a input to the drive, it will calculate thermal capacity utilised by the motor in terms of percentage using only motor terminal quantities. III. SIMULATION RESULTS The results are simulated in matlab and in DSP TMS320F28335 and results are captured. In matlab, results are plotted to show effect of loading and thermal time constant on motor temperature. DSP results are plotted in table which gives exact readings to validate. A. Matlab Simulation This section will illustrate the matlab simulation results through following cases. 1) Various Cases Following four graphs in each figure shows Thermal capacity utilised (TCU) by motor in terms of percentage, trip current setting Itrip (depends on speed), rms value of stator current and trip signal to motor respectively. If TCU % is >= 100 then this trip value sets to 1 and gives alarm. All rights reserved by www.ijsrd.com 825
[1] Consider a motor having rated current 8 A is supplied with reference speed 1500rpm, thermal time constant 1second, scope values update time interval is 0.1 second, with motor draws approximately 7.15 A (89% of FLC), Itrip setting = 1.05*speed trip current setting*rated current =1.05*1*8 = 8.4 A. Figure 3 shows this scenario. Here current drawn by the motor is less than trip current setting, so thermal capacity utilised by the motor increases and reach to its steady state value of 72.5 % after approximately five time constant and is less than 100%, hence motor will not trip. [4] Figure 6 shows motor is supplied with reference speed of 1000 RPM, with load applied to motor such that it draws 7.15 A (89% of FLC). But here Itrip=1.05*0.933*8 = 7.84 A for 1000 RPM. Here current drawn by the motor is less than trip current setting, so thermal capacity utilised by the motor less than 100% at steady state, hence motor will not trip. As RPM is less, cooling is slow compared to case 1. Steady state value of thermal capacity in this case (83 %) which is greater than case 1, it is due to the effect of cooling. Fig. 3: For loading of 89 % of FLC at 1500 RPM [2] Load applied to motor such that it draws approximately 8.65 A (110% of FLC) at 1500 RPM. Figure 4 shows this. Thermal capacity utilised by the motor increases and becomes greater than 100 %. Fourth graph shows tripping signal at 2.9 second. Fig. 6: For loading of 89 % of FLC at 1000 RPM [5] Figure 7 shows, motor is supplied with reference speed 750 rpm, with load applied to motor such that it draws 7.15A (89% of FLC). Here Itrip=1.05*0.9*8 = 7.56 A for 750 RPM. Here Thermal capacity utilised by the motor increases and reach to its steady state value (89%). Fig. 4: For loading of 110 % of FLC at 1500 RPM [3] Figure 5 shows load applied to motor such that it draws 10.5 A (130% of FLC) at 1500 RPM. Thermal capacity utilised by the motor increases and reaches to 100% at approximately 1.1 second and trip signal is given to motor. In this case motor will reach to its thermal limit earlier as compared to case 2 because current drawn by motor in this case is greater than in case 2. Fig. 7: For loading of 89 % of FLC at 750 RPM Comparing case 1, 4 and 5, we conclude that as speed is low, cooling is also slow. Hence steady state value of thermal capacity in this case (89 %) is greater than that in case 1 and 4. Hence the effect of cooling is clearly seen. [6] Figure 8 shows motor with rated current 8 A, speed vary from 1500 to 1200 to 700 to 1500 RPM, with thermal time constant 60 second. Following four graph shows Thermal capacity utilised (TCU) by motor in terms of percentage, trip current setting Itrip (depend on speed), rms value of stator current and trip value respectively. Highest steady state value of thermal capacity in this case is 10 %. Fig. 5: For loading of 130 % of FLC at 1500 RPM Fig. 8: For speed vary from 1500 to 1200 to 700 to 1500 RPM at thermal time constant of 60 second All rights reserved by www.ijsrd.com 826
[7] Figure 9 is simulated with all the parameters are same as in case 6, only thermal time constant is set to 5 second and results are captured. Fig. 9: For speed vary from 1500 to 1200 to 700 to 1500 RPM at thermal time constant of 5 second Here thermal time constant is set smaller than in case 6. Comparing 6 and 7 case, effect of thermal time constant is clearly seen. If thermal time constant is small then steady state value of thermal capacity will reach in less time. In this case motor will not trip but highest steady state value of thermal capacity is 78 %, whereas it is 10% in case 6. [8] Figure 10 shows motor with cyclic load condition of rated current 1 A, at 1500 RPM, with thermal time constant 1.5 second. Following four graph shows thermal capacity utilised (TCU) by motor in terms of percentage, trip current setting Itrip (depend on speed), rms value of stator current and trip value respectively. Third graph shows cyclic load varies between 1.1 to 0.8, cyclic load has rms current of 1 pu current. For a particular IEC class, thermal time constant value is fed, and DSP results are captured are as shown in the table 3, 4 and 5. Results are plotted for values of current upto 300% of full load current of motor. Since motor is operated by V/F drive, protection is given such that if inverter takes current above 300% of FLC setting, then it will trip signal. That s why current setting above 300% of FLC is not captured here. IEC Class 20 IEC Class 30 Thermal time constant 20*36 (=720 sec) FLC% IEC time required to trip (sec) Time taken by DSP algorithm to trip (sec) 300 120 95 250 180 140 200 320 232 150 740 486 Table 3: DSP result for IEC class 20 IEC time Thermal required time FLC% to trip constant (sec) 30*36 (=1080 sec) Time taken by DSP algorithm to trip (sec) 300 180 142 250 280 210 200 470 349 150 1150 728 Table 4: DSP result for IEC class 30 Table 3 and 4 concludes that, time taken by DSP algorithm to trip is approximately equal and less than actual IEC curves. Hence results are validating. Also another result is captured is DSP. Siemens motor datasheet is taken and is simulated in DSP. Thermal limit curve for Siemens motor (165 KW, 415 V, 1500 RPM) as shown in figure 12. From thermal limit curve, first IEC class is calculated from locked rotor time and locked rotor current values and then from obtained IEC class, thermal time constant is calculated as mentioned in previous section. Results are displayed in the table 5. Fig. 10: Cyclic load condition Highest steady state value of thermal capacity in this case is 98 %. Above cases explains the effect of thermal time constant and loading on thermal capacity (i.e. heat) accumulated by motor. B. DSP Results: Proposed method is simulated in DSP. IEC curve for various protection class is as shown in figure 11. Fig. 11: IEC curves Fig. 12: Thermal limit curve for 160 KW, 415 V, 1500 RPM Siemens motor Time taken by IEC Thermal time FLC% DSP algorithm to Class constant trip (sec) 20 20*36(=720 sec); Calculated from 300 95 250 140 All rights reserved by www.ijsrd.com 827
locked rotor 200 232 condition 150 486 Table 5: DSP result for 160 KW, 415 V, 1500 RPM Siemens motor Table 5 shows time taken to trip by DSP following thermal limit curves, hence this algorithm is validating for any of the motor. If motor trips, then thermal capacity accumulated by motor start to decay exponentially. Actual value of cooling thermal time constant is different from previously mention value of time constant because for stopped motor, no ventilation is available. Thermal capacity accumulated by motor will be accurately monitored provided that thermal time constant is accurate. Since thermal capacity used by the motor is monitoring continuously, it will help to know whether motor is able to start or not without any damage to it i.e. start inhibit condition. IV. CONCLUSION Thermal protection of induction motor technique is simulated in MATLAB/ Simulink and code is developed for prototype using TMS320F28335. Thermal capacity accumulated by the motor in terms of percentage is monitored continuously. From the monitored data, user can able to know, how much time the motor is allowed to run at a particular overload. Also user can know whether the motor is allowed to start or not if motor is in previously stopped condition i.e. start inhibit condition. User can monitor thermal condition of motor very easily. From the results, it is concluded that desired thermal protection is achieved. [8] Pinjia Zhang, Bin Lu, Thomas G. Habetler, Active Stator Winding Thermal Protection For Ac Motors, presented at the 2009 ieee ias pulp & paper industry conference, June 2009. [9] Mohamed I. Abdelwanis, F. Selim, An Efficient Sensorless Slip Dependent Thermal Motor Protection Schemes applied to Submersible Pumps, International Journal on Power Engineering and Energy (IJPEE), Vol. 6, No. 3, July, 2015. [10] Digital Signal Controllers, Texas Instruments data manual. [11] "Digital motor control Software Library", Texas Instruments, [12] IS/IEC Standard 60947-4-1:2000 [13] Stanley E. Zocholl, Optimizing Motor Thermal Models, Published in SEL Journal of Reliable Power, Volume 3, Number 1, March 2012. [14] Stanley E. Zocholl, Comparing Motor Thermal Models, Presented at the 59th Annual Georgia Tech Protective Relaying Conference, April, 2005. REFERENCES [1] "Report of Large Motor Reliability Survey of Industrial and Commercial Installations, Part II," Industry Applications, IEEE Transactions on, vol. IA-21, pp. 865-872, 1985. [2] "Report of Large Motor Reliability Survey of Industrial and Commercial Installations, Part I," Industry Applications, IEEE Transactions on, vol. IA-21, pp. 853-864, 1985. [3] "Report of Large Motor Reliability Survey of Industrial and Commercial Installations: Part 3," Industry Applications, IEEE Transactions on, vol. IA-23, pp. 153-158, 1987. [4] G. C. Stone, I. M. Culbert, and B. A. Lloyd, "Stator insulation problems associated with low voltage and medium voltage PWM drives," in Cement Industry Technical Conference Record, 2007. IEEE, 2007, pp. 187-192. [5] "Information Guide for General Purpose Industrial AC Small and Medium Squirrel-Cage Induction Motor Standards," NEMA Standard MG1-2003, August 2003. [6] Pero Ostojic, Renato Yabiku, Improving The Usage Of Temperature Sensors For Motor Thermal Protection, Petroleum and Chemical Industry Technical Conference (PCIC), Sept. 2012. [7] Temperature sensors: advantages & disadvantages. Application note td034. Temperature Product Group, October 2003. All rights reserved by www.ijsrd.com 828